We propose here a simple method to estimate carrier frequency offset (CFO) in a polarization-multiplexed coherent OFDM (CO-OFDM) system. Compared to existing techniques, this method based on the determination of the shifted position of the last filled OFDM sub-carrier (located just before the zero or empty sub-carriers area) permits to easily compensate large CFO, while not affecting the system robustness when facing chromatic dispersion, first and second order PMD.